Throughout the following, the term “optical fiber” shall be understood as a fibrous dielectric body comprising a first region with comparatively high refractive index (so called “core”) and a second region with comparatively low refractive index (so called “cladding”), the second region surrounding the first region. In addition, the cladding can be surrounded by an outer protection layer (so called “coating”). An optical fiber is optimized for low-loss guiding of electromagnetic radiation in the ultraviolet, visible or infrared regions.
FIG. 1a shows a schematic cross-section of an optical fiber 100 taken perpendicular to the fiber axis, the optical fiber 100 comprising a core 101, a cladding 102, a coating 103, and an interface surface 104 between the core 101 and the cladding 102. FIG. 1b shows the refractive index profile 110 of the optical fiber 100 corresponding to the cross-section shown in FIG. 1a. FIG. 1b shows the refractive indices n as a function of the radius of the optical fiber 100 of FIG. 1a as bar diagram (not to scale). The magnitude of the refractive indices n increases from left to right within the refractive index profile 110, as indicated by the arrow 111. The core 101 has a first refractive index n1 and, therefore, the core 101 is represented by a first bar 101a with great width. The cladding 102 surrounding the core 101 has a second refractive index n2 being smaller than the first refractive index n1 of the core 101. Therefore, the cladding 102 is represented in the refractive index profile 110 by two second bars 102a of medium width being adjacently positioned above and below the first bar 101a, respectively. The coating 103 enveloping the cladding 102 has a third refractive index n3 smaller than the second refractive index n2 of cladding 102 and is therefore represented by two third bars 103a of small width being adjacently positioned above and below the respective second bar 102a, respectively. As shown in FIG. 1b, at the interface surface 104 between the core 101 and the cladding 102 there is a refractive index step 104a between the first refractive index n1 of the core 101 and the second refractive index n2 of the cladding 102.
Guiding of electromagnetic radiation in the core 101 of the optical fiber 100 is achieved by total reflection at the interface surface 104 due to the refractive index step 104a. Usually, an electromagnetic ray is incident upon the interface surface 104 between the core 101 and the cladding 102 with an angle α1 being greater than the critical angle for total reflection αG, i.e. α1>αG is fulfilled, and is totally reflected there (α1 and αG are taken relative to the surface normal of the interface surface 104).
Throughout the following, the term “laser-active optical fiber” shall be understood as an optical fiber having the core doped with a laser-active material, for example with a rare earth composition. Absorption of optical energy (so called “pump light”) coupled into the optical fiber leads to a population inversion of the energy levels of the doping material in the core of the fiber, so that light amplification results for one wavelength or for several wavelengths. Laser-active optical fibers can be operated both as fiber lasers or fiber amplifiers.
Throughout the following, the term “photonic crystal fiber” shall be understood as an optical fiber which is internally structured namely usually by means of microscopically small holes in quartz glass. This structuring leads to a photonic band gap, so that light of a particular range of wavelengths is guided through the optical fiber.
Throughout the following, the term “fiber splice” shall be understood as a connection between two optical fibers which is not adapted to be separated and connected frequently, but is rather optimized as a permanent connection to show low radiation loss at the connecting position. Typically, the fiber ends are welded to each other, for example by means of a heat operation or an electric arc, to form a fiber splice.
The prior art discloses miscellaneous methods to draw off part of the light guided within an optical fiber (so called “tapping”) and to detect it. A plurality of these known methods is based on a mechanical modification of an optical fiber.
In this respect, U.S. Pat. No. 4,398,795 and EP 0619506 describe how the guided light can be tapped by fixing of an optical fiber and cutting into or polishing the fiber cladding. In EP 1014131 the use of a connecting piece made of light guiding material is taught, which connecting piece removes the coating and the cladding of an optical fiber so that the fiber core is uncovered and light can be coupled into the connecting piece.
In EP 1008876 is described a method for extracting light out of an optical fiber, wherein the optical fiber is impressed and deformed. For example, a wedge can be pressed onto the fiber for this. Due to the deformation, reflecting surfaces can be created in the optical fiber, which reflecting surfaces reflect a part of the light to a suitably positioned detector.
According to U.S. Pat. No. 4,781,428, a periodic spatial deformation of an optical fiber can be used in order to tap light out of the optical fiber. For this, the optical fiber can, for example, be pressed against a solid grid-like structure. At certain optical frequencies which are determined by the period of the grid mode-mixing is achieved, such as between core modes and cladding modes of the optical fiber. The cladding modes can be taped out of the optical fiber. The intensity of the decoupled light can be varied by varying the strength of the contact pressure.
Besides pure mechanical modifications of the fiber structure, the prior art also discloses methods which apply chemical techniques.
In U.S. Pat. No. 4,887,879 a tapping device is described, wherein at first cladding modes are induced in an optical fiber, which cladding modes are subsequently detected at a tapered position of the optical fiber. The tapering is produced, for example, by chemically etching away a part of the fiber cladding after removal of the cladding.
The prior art methods described above have one or several of the following disadvantages: The mechanical stability of the optical fiber is reduced due to the weakening of the fiber structure. Manufacturing of the described devices is often complicated, laborious and expensive, and involves the danger of unintentional damaging or destroying of the optical fiber, especially of the fiber core. Methods which comprise a mechanical or chemical narrowing of the fiber cladding further have the disadvantage, that the decoupled light is scattered preferably at very small angles with respect to the propagation direction. This complicates the light detection and/or necessitates complex assemblies for this.
A second group of methods employs a sufficiently tight bending of an optical fiber in order to extract a part of the light power. An example of one such “bending coupler” is described in U.S. Pat. No. 3,936,631. A device for monitoring the light power guided within an optical fiber is demonstrated in U.S. Pat. No. 5,080,506 wherein at first the modes propagating in the cladding are removed by applying a so called “mode stripper”, i.e. a material with a refractive index equal to or greater than that of the cladding. Subsequently, a part of the guided light power is coupled out by a bending coupler.
In U.S. Pat. No. 5,591,964 a battery driven or solar cell driven device for measuring the power guided within an optical fiber is described, wherein a part of the light power is removed from the optical fiber by applying a “micro-bend”. Such a micro-bend is produced by tightly bending the optical fiber at some position with a bending radius of preferably <2 mm, in doing so the temperature of the optical fiber is raised above the melting point for a short period of time due to a local heating. After cooling down, the micro-bend is mechanically fixed to the optical fiber. The micro-bend at the optical fiber leads to the effect that light rays reaching the core of the optical fiber in the region of the micro-bend are incident upon the interface surface between the core and the cladding of the optical fiber at an angle of α2 being smaller than the critical angle αG of total reflection, i.e. α2<αG is fulfilled (α2 and αG are taken relative to the surface normal of the interface surface). Therefore, in the region of the micro-bend the light rays are not totally reflected, but are partially decoupled out of the optical fiber through the cladding.
One disadvantage of such devices is the difficult reproducibility of the decoupled intensity, because the portion of decoupled power depends on the structure and composition of the respective optical fiber. In addition, the danger of a break of the fiber exists, unless laborious measures are taken to avoid it U.S. Pat. No. 5,039,188 describes a bending coupler with devices which apply compression pressure to an optical fiber in a suitable way, in order to reduce the risk of a break of the fiber). The extensive spatial distribution of the light radiated out presents another disadvantage of such devices. It is therefore difficult to collect this light on a small photo detector which affects the applicability of such couplers for fast controls.
Thermally manufactured micro-bends, as shown in U.S. Pat. No. 5,591,964, are critically mechanically stable and additionally include a manufacturing challenge because at least two parameters (temperature and bending radius) have to be monitored during manufacturing. This increases the risk of damaging the optical fiber, for example as a result of a too high or too long influence of temperature.
In another group of methods, the fiber is severed in order to tap a part of the light power. In U.S. Pat. No. 4,165,496 is described a beam splitter which is realized by exactly coaxial aligning two fiber pieces with skew beveled end surfaces being parallel to each other. At the end surfaces, light is reflected out of the optical fiber. This method, however, requires an extremely precise and therefore laborious adjustment. In addition, there is the risk that mechanical agitations cause an offset of the fiber pieces, thereby disconnecting the light guide.
Advancements of this idea include the production of joint positions by splicing of fiber pieces. In U.S. Pat. No. 4,475,789 a fiber-optic power monitor is demonstrated, wherein two optical fibers having different mode volumes are spliced together. Thereby, the mode volume of the second optical fiber is smaller than that of the first optical fiber, such that a part of the light guided through the first optical fiber cannot propagate through the second optical fiber. The scattered light resulting at the splice position is detected and serves as control signal for a power control.
The method described in U.S. Pat. No. 4,165,496 requires optical fibers with different mode volumes, for example optical fibers with different refractive index profiles are applied. However, in case of single-mode waveguides—in particular for less common wavelengths—suitable optical fibers with different refractive index geometry are difficult to obtain, if at all. This method is further disadvantageous for the use of multi-mode waveguides when the ray profile or the transverse mode structure, respectively, of the light source changes, for example due to power variations of a laser light source or due to a mechanical impact onto the optical fiber (for example as a result of touching). In this case, the ratio of the light power guided in the two optical fibers may change. This method is therefore susceptible for errors concerning its ability to measure the power guided in the second optical fiber.
In U.S. Pat. No. 4,475,789 a method is taught, wherein an optical fiber is severed, and one of the resulting end surfaces is coated within a vacuum apparatus with dielectric material (e.g. TiO2) or a metal (e.g. Ti). Subsequently, the fiber pieces are spliced with each other again. While doing so, electric arcs are applied until a desired reflectivity is obtained at the connection position.
Major disadvantages of this method are the technical manufacturing effort for specially manufactured optical fibers due to the vacuum coating, and the accompanying apparatus costs. For the manufacturing of these specially manufactured optical fibers, in particular an optical fiber is severed perpendicular to the fiber axis, such that two fiber ends result. In a vacuum apparatus, the end surface of one of these fiber ends is coated with a suitable refractive material or reflective material, respectively. Thereafter, the coated and uncoated fiber-ends are spliced together at their end surfaces. The manufacturing therefore requires at least one laborious vacuum coating process. Further, there exists the risk that the transmission characteristics of such an optical fiber are deteriorated due to unwanted absorption by this refractive material or reflective material, respectively.
Furthermore, other methods apply doping of the optical fiber with extrinsic atoms or particles in order to decouple a part of the light by reflection or refraction. According to U.S. Pat. No. 4,923,273, scattered light can be generated by utilizing means (e.g. chemical admixtures) incorporated into the optical fiber, which means modify one or more fiber parameters. This publication refers in particular to “activatable means”, for example extrinsic atoms evoking a change of refractive index (or another fiber parameter) due to the impact of electromagnetic radiation, due to heat, or due to bombardment with electrons or ions.
A disadvantage of these devices consists in increased material costs that arise as a result of the use of specially manufactured optical fibers. In addition, there is the risk that as a result of undesired absorption in such specially manufactured optical fibers the transmission properties of such an optical fiber are deteriorated.
The application of light scattering or light refracting particles in the core of a light guide is described in U.S. Pat. No. 4,618,211. If the particles are incorporated during the manufacturing of the fiber, the optical fiber emits scattered light along its whole length. Alternatively, at first the fiber core is formed which is treated by means of heat or radiation in order to generate light deflecting defects. Or the optical fiber is irradiated by ionizing radiation or laser light after completion, thereby provoking microscopic deficiencies in the structure of the fiber core.
If the particles are incorporated during manufacturing of the fiber, the disadvantage of high damping of the guided light occurs here as well. In addition, the relative portion of the decoupled light power is difficult to control. On the other hand, in the publication it is pointed out, that the possibilities mentioned therein are not well suited to incorporate scattering centers only after completion of the optical fiber.
In U.S. Pat. No. 4,466,697 an optical fiber is described, whose core is interspersed with light scattering particles as scatter centers. These light scattering particles can be incorporated into the core by admixing adequate material in the melt from which the core of the optical fiber is drawn, and by spraying adequate material onto the core, before creation of the cladding.
A device and a method to monitor the light guided through an optical fiber are known from DE 4313795. Therein an optical fiber is enveloped by a glass tube in the vicinity of a joint position (but not at the joint position itself). This glass tube is filled with an adhesive and guides scattered light to a detector, which scattered light is radiated out of the core of the optical fiber into the cladding at the joint position and is transferred from there into the glass tube.
From DE 4314031 a means for monitoring and protecting optical waveguide (OWG) cables is known, said means detecting energy leakage on account of a malfunction out of an OWG cable or an OWG fiber. The damage of the OWG cable occurs due to uncontrolled decoupling of the laser rays at defects of the OWG cable, for example based on a minor case of falling below the allowed bending radius.
A method for monitoring a splicing process when splicing two optical fibers to each other is described in JP 55035350. Shortly prior to the splicing operation, light is transmitted through the two optical fibers. The scattered light emanating at the position to be spliced is detected by a detector and the two optical fibers are adjusted relative to each other such that the detector does not detect scattered light any more. In this way, an optimal adjusting of the two optical fibers relative to each other is obtained, i.e. the two optical fibers are spliced to each other only if an optimal transmission of light through the two optical fibers has been reached.
In U.S. Pat. No. 4,371,897 a spatially quantitative light detector is disclosed comprising an optical fiber and a photo sensor coupled to the optical fiber. The core of the optical fiber is interspersed by a fluorescing substance. Light incident upon the surface of the optical fiber is guided through the optical fiber to the photo sensor and is detected there by the photo sensor.